A pipe of pipeline is usually laid beneath the surface of the ground, buried at a location between the walls of the adjacent buildings, or arranged in a place readily inaccessible by a maintenance personnel. There sometimes occurs an abnormal spot or an injury on the pipe upon impact from an external force or a leakage of an internal fluid resulting from a corroded pipe wall. It is very cumbersome or difficult to detect occurrence of abnormality on the pipe and locate it once the pipe has been buried or laid beneath the ground surface.
Published Unexamined Japanese Patent Application No. 60-49199 discloses a piping buried with an optical fiber spirally wound thereon. This optical fiber is connected at one end to a light emitting element and at the other end to a light receiving element, time differentiating circuit, alarm circuit, etc. A light signal emitted from the light emitting element is transmitted via the optical fiber to the light receiving element where it is converted to an electric signal, which is differentiated by the time differentiating circuit. An external lateral force on the buried piping causes microbends on the optical fiber. This leads to a greater transmission loss across the full length of the optical fiber and an abrupt variation in the output signal of the time differentiating circuit. The occurrence of abnormality is thus detected.
A piping, if newly buried beneath the ground surface, may be so done at a desired location with the wound optical fiber disposed on the piping. If, on the other hand, an optical fiber cable is provided on an already buried piping, digging-out has to be done at a location around the piping, involving an unpractically larger construction work expense.
The digging is effective to the situation in which the piping is deformed over a certain ground area due to the occurrence of earthquake, local ground subsidence or upheaval, etc., but a local impact by an excavating machine, etc., on the piping and resultant occurrence of a injured spot or spots on the piping do not correspond to the location of the optical fiber cable, sometimes failing to positively detect the occurrence of abnormality on the optical fiber.